A Study On Cadmium Releasing And Solid-liquid Equilibrium In Soils Of Chengdu Suburban Area Under Acid Rain Condition

Acid rain may not only affect plant growth, but it may increase the solubility ofdetrimental heavy metals previously fixed in the soil and then increase the bioavailability ofheavy metals which can be absorbed by plants quite easily. This will do harm to the foodchain and the underwater pollution will occur through the leaching downward of heavy metals.Chengdu city has been plot as a national acid rain controlling area from 2004 and the acid rainpollution shows an increasingly serious trend during these several years. What's more,cadmium content in Chengdu agricultural soils is on high side, and in some parts, the valueseven exceed the third standard, according to national soil environmental quality. Although thefact of high soil Cd content has evoked people attention, the behaviors such as chemicalspeciation, activation and transferring still haven't been studied clearly. Furthermore, underacid rain, the impact mechanism and degree of Cd activation and releasing in Chengdu soil.To solve the pollution problems efficiently, the most important thing is to make clear the theimpact mechanism and degree of Cd activation and releasing under acid rain and thesolid-liquid equilibrium of soil Cd. Unfortunately, study results in these crucial fields are stillsuspending especially on the later one, there's no any achievements so far. To solve theproblems mentioned above, we carried on the research.We focused on cadium, a heavy metal in two major agricultural soil (paddy soil and yellowsoil) in suburban aera of Chengdu city. According to the precipitation feature in Chengdu,acid rains were simulated through considering various pH values and concentrations of anionsand cations in order to take the leaching experiments in lab. We determined the crucial factorsin acid rain affecting Cd solubility in soil through detecting Cd content and the pH values ofleachates. Based on the results, we studied the solid-liquid phase equilibrium characteristicsofquarternary system Cd²⁺, Na⁺//Cl^-, SO₄^2- under 298K, which is in an attempt to review thebalance status of soil Cd within the system. The research results could reflect impersonallyCd's activation and fixation in surface eco-system under natural conditions, and could be usedin predicting Cd's environmental geochemical behaviors in soil-water system under actions ofnatural and anthropogenic effects as well. The results also could be put into many other usages to study other heavy metals biogeochemical behaviors in surface biogeochemicalsystem.The results of this paper indicate that, generally speaking, acid rain accelerates the releaseof soil Cd. It seems to different types of soils under the same acid rain condition, the releasespeed of Cd and maximum release content are correspondingly different. There exists evidenteffects of acid rain pH, concentrations of anions and cations on the release of Cd from thesoil.We adopted six existed dynamics equations and dynamics equations put forward byourselves to fit the data obtained from leaching test. Comparing with the six existed equations,the dynamics equation established by ourselves can descript soil Cd release procedure underthe impact of acid rain very well, and polynomial euqation is the best one among those othersix equations. Furthermore, soil Cd release can be divided into two steps according to theprocedure feature, the fast one and the slow one, we adopted six dynamics equations to fit theexperimental data, and confirmed the fittest equation to descript the two steps separately.Tostudy the solid-liquid equilibrium of Cd-bearing system in the soil under acid rain and otherenvironmental factors, and to provide scientific evidence for anticipate Cd bioavailability,adopting isothermal solubility equilibrium method, we carried out the study on phaseequilibrium and phase diagram of the quaternary system Cd²⁺, Na⁺//Cl^-, SO₄^2- at 298K.Different salts have salting-out effect on each other. There do exist certain laws betweenphysio-chemical properties of the solution and the solution concentration and solutioncomposition. Valuable research results were obtained by analyzing those diagrams and thepredicted aim was reached.